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  <p class="align-right"><strong>directive</strong></p>
<section id="op-3">
<span id="directives-op-3"></span><span id="index-0"></span><h1><code class="docutils literal notranslate"><span class="pre">op/3</span></code><a class="headerlink" href="#op-3" title="Link to this heading"></a></h1>
<section id="description">
<h2>Description<a class="headerlink" href="#description" title="Link to this heading"></a></h2>
<div class="highlight-logtalk notranslate"><div class="highlight"><pre><span></span><span class="k">op</span>(<span class="nv">Precedence</span>, <span class="nv">Associativity</span>, <span class="nv">Operator</span>)
<span class="k">op</span>(<span class="nv">Precedence</span>, <span class="nv">Associativity</span>, [<span class="nv">Operator</span>, ...])
</pre></div>
</div>
<p>Declares operators. Global operators can be declared inside a source file
by writing the respective directives before the entity opening directives.
Operators declared inside entities have local scope. Calls to the standard
term input and output predicates take into account any locally defined
operators.</p>
</section>
<section id="template-and-modes">
<h2>Template and modes<a class="headerlink" href="#template-and-modes" title="Link to this heading"></a></h2>
<div class="highlight-logtalk notranslate"><div class="highlight"><pre><span></span><span class="k">op</span>(<span class="o">+</span>integer, <span class="o">+</span>associativity, <span class="o">+</span>atom_or_atom_list)
</pre></div>
</div>
</section>
<section id="examples">
<h2>Examples<a class="headerlink" href="#examples" title="Link to this heading"></a></h2>
<p>Some of the predicate argument instantiation mode operators used by Logtalk:</p>
<div class="highlight-logtalk notranslate"><div class="highlight"><pre><span></span><span class="p">:- </span><span class="k">op</span>(<span class="m">200</span>, fy, <span class="o">+</span>).
<span class="p">:- </span><span class="k">op</span>(<span class="m">200</span>, fy, <span class="o">?</span>).
<span class="p">:- </span><span class="k">op</span>(<span class="m">200</span>, fy, <span class="o">@</span>).
<span class="p">:- </span><span class="k">op</span>(<span class="m">200</span>, fy, <span class="o">-</span>).
</pre></div>
</div>
<p>An example of using entity local operators. Consider the following <code class="docutils literal notranslate"><span class="pre">ops.lgt</span></code>
file:</p>
<div class="highlight-logtalk notranslate"><div class="highlight"><pre><span></span><span class="p">:- </span><span class="k">initialization</span>((<span class="k">write</span>(<span class="o">&lt;=&gt;</span>(<span class="m">1</span>,<span class="m">2</span>)), <span class="k">nl</span>)).

<span class="p">:- </span><span class="k">object</span>(ops).

<span class="p">    :- </span><span class="k">op</span>(<span class="m">700</span>, xfx, <span class="o">&lt;=&gt;</span>).

<span class="p">    :- </span><span class="k">public</span>(w<span class="o">/</span><span class="m">1</span>).
    w(<span class="nv">Term</span>) <span class="o">:-</span>
        <span class="k">write</span>(<span class="nv">Term</span>), <span class="k">nl</span>.

<span class="p">    :- </span><span class="k">public</span>(r<span class="o">/</span><span class="m">1</span>).
    r(<span class="nv">Term</span>) <span class="o">:-</span>
        <span class="k">read</span>(<span class="nv">Term</span>).

<span class="p">:- </span><span class="k">end_object</span>.
</pre></div>
</div>
<p>Loading the file automatically calls the initialization goal. Compare its
output with the output of the <code class="docutils literal notranslate"><span class="pre">ops::w/1</span></code> predicate. Compare also reading
a term from within the <code class="docutils literal notranslate"><span class="pre">ops</span></code> object versus reading from <code class="docutils literal notranslate"><span class="pre">user</span></code>.</p>
<div class="highlight-text notranslate"><div class="highlight"><pre><span></span>| ?- {ops}.
&lt;=&gt;(1,2)
true.

| ?- ops::w(&lt;=&gt;(1,2)).
1&lt;=&gt;2
true.

| ?- ops::r(T).
|: 3&lt;=&gt;4.

T = &lt;=&gt;(3, 4).

| ?- read(T).
|: 5&lt;=&gt;6.

SYNTAX ERROR: operator expected
</pre></div>
</div>
<div class="admonition seealso">
<p class="admonition-title">See also</p>
<p><a class="reference internal" href="../methods/current_op_3.html#methods-current-op-3"><span class="std std-ref">current_op/3</span></a></p>
</div>
</section>
</section>


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